Zubin -- this looks very interesting and I wish you lots of success. A friend of mine recently suggested we should build such a service and I am glad to report back to him, someone has ("glad" because I was already working on my own start-up). I would suggest allowing for instantaneous deletes as well -- so as soon as the consumer opens the message, poof, it is gone -- one-time only. That was his use case scenario and we riffed on that for quite some time.
Cheers, Seth Cohen Screenius, Inc.